The present invention relates in general to steering systems for materials handling vehicles and, more particularly, to a steer-by-wire power steering system wherein the steered wheel or wheels and the steering wheel or tiller are synchronized with one another and a virtual coupling force is exerted on the steering tiller to maintain synchronization.
In existing steer-by-wire steering systems used on materials handling vehicles, some resistance is provided against rotation of the steering tiller. However, the steering tiller resistance is not related to the speed of rotation of the tiller or whether the tiller advances beyond the position of the steered wheel(s). The tiller and the steered wheel(s) seem to be coupled as long as the operator does not outrun the steered wheel(s), i.e., the operator does not move the tiller so rapidly that the steered wheel(s) cannot keep up with the movement requested by the tiller. If the tiller is turned too rapidly and outruns the steered wheel(s), there is no feedback to the operator indicating that the steering tiller is outrunning the steered wheel(s) and the steered wheel(s) is not synchronized with the tiller position.
It is desired to make the steer-by-wire steering on materials handling vehicles more closely resemble the properties of a mechanically coupled steering system.
In accordance with the present invention, to provide synchronization or coupling of a steering tiller of a materials handling vehicle and the steered wheel(s), even though the two are not mechanically coupled, a motor or brake is associated with the steering tiller and used to provide a force representative of a difference between the steering position selected by the steering tiller and the actual position of the steered wheel(s). Absolute position monitoring is provided for both the steering tiller and the steered wheel(s) with outputs from the associated absolute position monitors being provided to a controller which is programmed to operate the motor or brake to oppose rotation of the tiller if the steering tiller is rotated too rapidly, i.e., at a rate that is faster than the steered wheel(s) can be turned, and if the steered wheel(s) reach a mechanical stop. The steering system of the present invention thus simulates mechanically coupled steering systems and also enables a variety of startup scenarios to take place. For example, an operator can select a desired steering tiller position to correspond to the straight ahead steer position for the vehicle.